1. Field of the Invention
The present invention relates to a method of manufacturing a shutter to be slidably put on a protective envelope, formed by folding a metal sheet, such as a stainless steel sheet, in a shape having a U-shaped cross section.
Description of the Prior Art
Referring to FIG. 4, a conventional protective envelope (3.5 in. MFD) 1 is formed by molding a synthetic resin, such as ABS resin, and contains a 3.5 in. flexible disk 2, i.e., a recording medium having the shape of a disk. The protective envelope 1 has opposite side walls la provided with a pair of slots 3, respectively, and a shutter 4 formed by folding a metal sheet, such as a stainless sheet, in a shape having a U-shaped cross section, is put on the protective envelope 1 so as to slide in the directions of the arrows a along the outer surfaces of the side walls 1a of the protective envelope 1 to close or open the slots 3 formed in the side walls 1a of the protective envelope 1.
Referring to FIG. 5, in manufacturing the conventional shutter 4, a flat metal strip 7 is unwound from a strip coil 6 of metal strip, such as a stainless strip, a blank 8 having the shape of the development of the U-shaped shutter 4 is punched out from the flat metal strip 7, and then the blank 8 is folded in the U-shape to complete the shutter 4. The shutter 4 has opposite side walls 41 and an end wall 42 connecting the opposite side walls 41.
When the shutter 4 is thus manufactured by the conventional method comprising steps of punching out the blank 8 from the flat metal strip 7 and folding the blank 8 in the U-shape, the opposite side walls 41 of the shutter 4 are liable to diverge and the width W1 of the shutter 4, i.e., the distance between the edges 41a of the opposite side walls 41, becomes excessively large as shown in FIG. 6(A) or the opposite side walls 41 of the shutter 4 are liable to converge and the width W1 becomes excessively small as shown in FIG. 6(B). If the width W1 of the shutter 4 is excessively large, the edges 41a of the opposite side walls 41 of the shutter 4 rise from the outer surfaces of the side walls 1a of the protective envelope 1 and collide against adjacent parts of a recording/reproducing apparatus in loading the protective envelope into or unloading the same from the recording/reproducing apparatus. If the width W1 of the shutter 4 is excessively small, the edges 41a of the opposite side walls 41 of the shutter 4 press hard on the side walls 1a of the protective envelope 1 to abrade the side walls 1a of the protective envelope 1 when the shutter 4 slides along the side walls 1a of the protective envelope 1. When the side walls 1a of the protective envelope 1 are abraded, powder of the synthetic resin forming the protective envelope 1 is produced and the powder enters the interior of the protective envelope 1 to soil the flexible disk 2, which causes drop-out in recording information on and in reproducing recorded information from the flexible disk 2.